1. Field of the Invention
The present invention relates to an apparatus for treating waste water containing a phosphorus compound such as phosphoric acid or phosphate ions and a nitrogen compound such as organic nitrogen, nitrite nitrogen, nitrate nitrogen and ammonia nitrogen.
2. Description of the Related Art
It has heretofore been well known that existence of a nitrogen compound and a phosphorus compound is one of causes of eutrophication of rivers and lakes. These phosphorus compound and nitrogen compound exist in domestic waste water and industrial waste water in large amounts and are difficult to purify, and currently no effective measures can be taken against these compounds. In general, to treat waste water containing the nitrogen compound, biological treatment is implemented. The biological treatment comprises two steps, i.e., a nitrification step of converting ammonia nitrogen to nitrate nitrogen and a denitrification step of converting nitrate nitrogen to a nitrogen gas.
Meanwhile, a variety of techniques for treating the phosphorus compound have been proposed, and a lime agglomeration/precipitation technique is known for domestic waste water. This is a technique for removing phosphate ions in water to be treated (hereinafter, “water to be treated” will be referred to as “for-treatment water”) by causing the phosphate ions to react with calcium ions so as to agglomerate and precipitate the phosphate ions as calcium hydroxyapatite which is a water-insoluble salt.
However, an apparatus used for treating the nitrogen compound and the phosphorous compound in the conventional biological treatment has problems that it requires two independent reaction vessels and that because the treatment proceeds slowly, the efficiency of the treatment is low. Further, the conventional technique has a problem that it requires a large-capacity apparatus since it does not treat a nitrogen compound and a phosphorus compound which are contained in for-treatment water simultaneously.
Further, in the biological treatment, there is another problem that large-capacity aerobic and anaerobic vessels are required for keeping nitrifying bacteria and denitrifying bacteria, thereby inducing an increase in equipment construction costs and an apparatus installation area. There is still another problem that since the denitrifying bacteria are significantly influenced by an ambient temperature environment, components contained in for-treatment water and the like, and in particular, during the winter season when the temperature is low, their activities and denitrifying action are lowered, resulting in unstable treatment efficiency.
In addition, in the above technique for treating the phosphorus compound, since the pH of the for-treatment water after the condensation/precipitation of the phosphorus compound is high, the alkaline for-treatment water resulting from the treatment of the phosphorus compound must be neutralized. Further, the technique also has a problem that maintenance is difficult due to a large amount of lime used.